There are slight restrictions on Class G (including a MATZ) being uncontrolled.
If you are in communication with a controller (including for a Basic Service) then controllers can issue instructions. From CAP 493 (Manual of Air Traffic Services):
Instructions issued by controllers to pilots operating outside controlled airspace are not mandatory; however, the services rely upon pilot compliance with the specified terms and conditions so as to promote a safer operating environment for all airspace users.
Although IFR/VFR flight within Class F/G airspace outside the ATZ is permitted without an ATC clearance, controllers will act on the basis that pilots will comply fully with their instructions in order to promote a safer operating environment for all airspace users.
As I understand it, a controller can give instructions and expect a pilot to comply
unless the pilot says otherwise. That is, the instruction isn't mandatory but you should say if you're not going to comply.
Libertarian enthusiasts might think that is a good reason not to talk to anyone, but I'm quite prepared to follow instructions for safety or say if I'm not going to.